The present invention relates to a control system for an automatic transmission of a vehicle and, more particularly, to a control system for an automatic transmission, which is equipped with an auxiliary transmission unit capable of changing two high and low stages and adapted to be set to the high gear stage at a reverse stage.
In an automatic transmission of a vehicle, there is frequently used an overdrive system for improving the fuel economy and the calmness, as is well known in the art. On example is disclosed in Japanese Patent Laid-Open No. 157165/1993, in which an overdrive system including a set of planetary gear mechanism is disposed at the input side of a main transmission unit for setting forward four and reverse one stages. This overdrive system is constructed to set a low gear stage (or a directly connected stage), in which the entirety makes one rotation, by connecting the carrier and the sun gear of the planetary gear mechanism through a clutch, and to set a high gear stage by fixing the sun gear through a brake. In the automatic transmission disclosed in the above-specified Laid-Open, moreover, the overdrive system is set to the high gear stage not only for setting the highest gear stage, i.e., the 5th speed but also for setting the reverse stage, so that the reverse stage may be prevented from taking a high gear ratio.
After all, the automatic transmission, as disclosed in the above-specified Laid-Open, can set forward five and reverse one stages and is equipped therefore with first to third solenoid valves for controlling the shift. As in the ordinary automatic transmission, moreover, the engine braking is required for the 1st or 3rd speed. For this requirement, the aforementioned automatic transmission is further equipped with a fourth solenoid valve for feeding a signal pressure to a predetermined change-over valve to set the engine braking state selectively.
In order to set the individual gear stages in the drive range and in the engine braking range, the aforementioned control system for the automatic transmission of the prior art is equipped with the first to fourth four solenoid valves. For the entire compactness of the hydraulic control system or the automatic transmission, however, it is desirable to reduce the number of solenoid valves. In this case, the high gear stage of the overdrive system is set at the 5th speed and the reverse stage, whereas the engine braking state is set at the gear stages of the 1st speed and the 3rd speed. It is, therefore, conceivable to commonly use the solenoid valves which are used at the gear stage having no mutual interference, as called so. Specifically, it is conceivable to cause the solenoid valves for setting the overdrive system to the high gear stage to set the engine braking at the gear stages such as the 1st speed or the 3rd speed in the engine braking range.
In case, however, the solenoid valve for the so-called "overdrive" is used commonly as the solenoid valve for the engine braking, no engine braking is effected at the 1st speed in the drive (D) range. Thus, the acting states of the solenoid valve are reversed at the reverse stage setting time and at the 1st speed of the D-range. Specifically, this solenoid valve is a valve for changing the overdrive system to the high gear stage and is usually constructed to output a signal pressure, when in a deenergized (OFF) state, from the standpoint of a fail-safe operation for setting a high gear stage at the time of a failure. Hence, the solenoid valve is turned OFF, when the reverse stage is set, but ON to output no signal pressure at the 1st speed of the D-range, in which the engine braking is ineffective.
Thus, if the aforementioned solenoid valve is simply used commonly for setting the overdrive and for the engine braking, the energization patterns for the solenoid valve are reversed between the reverse stage and the D-range. As a result, in case the detection signal of the reverse stage (i.e., the R-range) is outputted by an electric failure although the 1st speed of the D-range is set, the aforementioned solenoid valve is turned OFF to output the signal pressure so that the engine braking is effective at the 1st speed of the D-range. Specifically, a multi-disc brake in parallel with a one-way clutch to be applied at the 1st speed is applied so that the shift cannot be smoothly executed to deteriorate the shift shock, or the braking state may be invited as soon as the accelerator pedal is released, to deteriorate the drive feeling.